1. Field of the Invention
This invention relates to objective lenses of large relative aperture which are capable of high grade imaging performance throughout a focusing range from infinity to extremely short object distances.
2. Description of the Prior Art
General purpose photographic objectives are corrected for aberrations to provide the best performance when focused at infinity. On the other hand, macro objectives which are used principally for close-up photography have to take into account the fact that aberrations vary with object distances. Generally, in planning, a reproduction ratio of 1:10 or thereabouts is selected as the starting point at which the best optical performance is obtained. However, in many cases, when it comes to objectives of relatively large apertures or wide angle objectives, the range of variations of aberration increases as the reproduction ratio increases. The spherical aberration and asymmetrical aberration of extra-axial rays of light deteriorate remarkably. This is because the paraxial pencil from a close object tends to diverge more strongly than an infinitely distant object so that it is near the stop that the paraxial pencil transits to divergence. Therefore, the spherical aberration of the positive lens group that follows the stop becomes inadequate for effective compensation. Also, since the principal ray of the extra-axial pencil forms a smaller angle with the optical axis when focusing down to shorter distances, outward coma is often produced. Such large variations of the aberrations with different reproduction ratios makes it meaningless to optimize the aberration correction at the reproduction ratio of 1/10 or thereabouts when the object distance is infinite or very close because if fails to preserve high optical performance. In order to prevent such loss in the imaging performance due to the change of the reproduction ratio, it has been a common practice to impart a floating movement into part of the optical system. Objectives attempting to use such a system are disclosed in U.S. Pat. No. 3,884,557, Japanese Laid-Open Patent Applications Sho 52-7723, 53-10425 and 55-28038. However, such conventional methods are complicated and incapable of providing the desired improvement in performance, and permit the interval between the principal points of the lens group to be widened enough so that the focal length varies extensively.